The Danish Adoption Register will be used in an attempt to determine the relative contributions of heredity and environment to obesity and thinness. From 3640 adoptees, whose height and weight have been determined, we will draw samples of 400 men and 400 women divided equally among four weight categories: thin, normal weight, overweight and obese. Measures of their degree of adiposity will be compared with those of their half siblings who had been raised by their (the adoptees') biological parents. The relationship between their adiposity and that of their adoptive and biological parents will also be compared by means of analysis of variance, path analysis and family set method.